It is known that tractor-trailer units often have steering difficulties, especially when the trailer is long and the road narrow, causing the rear wheels of the trailer to either hit the corner of the sidewalks or go over to corner ditch, thereby causing the trailer to dangerously tilt sideways, when not tilting over and falling off.
In order to avoid the above problems, most of the tractor-trailer units need to take much wider turns, whenever possible, and go over opposite side traffic lanes that may cause road accidents. Also, the tractor-trailer units need to go through much longer road paths to avoid any narrow roads to get to their final destination, thus increasing travel time and reducing the efficiency of the cargo delivery network.
To obviate the above problems, many mechanisms provide for a self steering of the rear wheel train. Such mechanisms, as shown in U.S. Pat. No. 3,930,669 to Kollander et al, U.S. Pat. No. 5,197,751 to Rankel, U.S. Pat. No. 5,213,353 to Williams and U.S. Pat. No. 5,246,242 to Penzotti, are expensive and require special rear wheel trains since they are built-in the latter. Also, in case of breakage of the mechanism, the rear wheel train must be fixed before the trailer can move again, thus causing more problems of towing the trailer away for repair.